Embodiments and aspects of the invention are generally directed to methods and associated apparatus for rapid and accurate nanoscale metrology. More particular embodiments and aspects are directed to methods and associated apparatus for rapidly and accurately measuring and/or characterizing nanoscale features of integrated circuits and the like. Most particularly, embodiments and aspects are directed to methods and associated apparatus using focused beam scatterometry for rapidly and accurately measuring and/or characterizing one or more nanoscale-related process errors of an object having a periodic or quasi-periodic structure such as, but not limited to, semiconductor metrology.
Accurate measurement of subnanometer features is central to the success of semiconductor fabrication. For most fabrication lines, inline process monitoring is essential to assuring that etch depths are accurate, side wall angles are within tolerance, coatings have the right thickness and composition, critical dimensions (CD) are accurate, and that line edge roughness is sufficiently small. For features smaller than 100 nm, direct imaging is usually not possible unless carried out in a scanning electron microscope. The measurements are therefore carried out with test targets whose features are similar to the circuit features but whose scattering properties can be predicted using numerical tools such as rigorous coupled wave analysis.
Scatterometry, involving the retrieval of a grating shape from the measurement of scattered light, has become the standard method for deducing nm-scale deviations from nominal values in semiconductor lithography. Most approaches to scatterometry have measured the scattering from oblique angles (and often over several wavelengths) in a sequential manner. The beam is usually brought to a slow (low NA) focus on a test target whose nominal dimensions are known; the results of the measurement are compared to a lookup table generated from a rigorous electromagnetic model.
A number of innovative extensions to conventional scatterometry have evolved in recent years. These have been aimed at features 22 nm and smaller, and are also driven by the need to measure the details of deep trench structures, and novel gate designs such as are found in Fin FET devices. Some reported approaches combined very precise optomechanical scanning with differential imaging or have explored phase-sensitive scatterometry using coherent light (e.g. digital holographic microscopy).
Scatterometry methods may be compared with conventional and spectroscopic ellipsometry. Ellipsometry is a model-based measurement method that combines rigorous electromagnetic theory with polarization control and analysis to decouple film thickness from the optical constants of the material. Micro-ellipsometry (in which the polarization over the pupil of a focusing lens is controlled and analyzed) allows ellipsometry to be carried out at very high spatial resolution but is generally limited to feature sizes larger than a wavelength.
The inventors have recognized the benefits and advantages in providing enabling solutions using polarization in the scatterometry of deeply subwavelength features that have known nominal values. These solutions enable optimizing the polarization distribution over the pupil in order to extract several process parameters in a single measurement. The embodied invention particularly advantageously enables the decoupling of multiple process errors using an off-null measurement method and apparatus.